This invention relates in general to a receiver circuit and, more particularly, to a method and apparatus for minimizing image power in the output of a receiver circuit.
In receiver circuits which process radio frequency signals, it can be important to provide efficient image rejection, or in other words to ensure that each output from the receiver has minimum image power. One known image-rejection receiver includes a first modulation stage which has two mixers that each modulate a radio frequency input signal using a respective one of two modulating signals from a first oscillator, and a second modulation stage which has four mixers that each modulate the output of one of the mixers of the first stage using one of two modulating signals produced by a second oscillator. Two combining circuits then combine the outputs of respective pairs of the mixers in the second stage, in order to produce I and Q output signals from the receiver circuit.
Each of the two oscillators in this circuit has a phase adjustment portion, which allows adjustment of the phase difference between the two modulating signals produced by that oscillator. By appropriate adjustment of the phase difference in each oscillator, the two modulating signals from that oscillator can be maintained in phase quadrature with each other, which in turn results in minimum image power in each of the I and Q outputs, so that receiver circuit has good image rejection.
In this known circuit, however, the first oscillator operates a much higher frequency than the second oscillator, for example about five times the frequency of the second oscillator. It is known that, as the operating frequency of an oscillator increases, it becomes progressively more difficult to design phase adjustment circuitry which can accurately adjust a phase difference between two outputs from the oscillator. Accordingly, in this known type of receiver circuit, it is difficult and complex to design suitable phase adjustment circuitry for the first oscillator, which operates at a much higher frequency than the second oscillator.
From the foregoing, it may be appreciated that a need has arisen for a method and apparatus which permit a receiver system to minimize image power in an output, while avoiding the need to design phase adjustment circuitry for a high-frequency oscillator. According to the present invention, a method and apparatus are provided to address this need, and involve: generating first and second modulating signals at a first frequency and with a phase difference; generating third and fourth modulating signals at a second frequency and with a phase difference; generating fifth and sixth modulating signals at a third frequency and with a phase difference; generating intermediate signals in a first stage of modulation by using the first and second modulating signals to modulate an input signal; generating intermediate signals in a second stage of modulation by selectively using the third, fourth, fifth and sixth modulating signals to modulate the intermediate signals from the first stage of modulation; generating first and second output signals which are each a function of selected intermediate signals from the second stage of modulation; and controlling the phase difference between the third and fourth modulating signals and the phase difference between the fifth and sixth modulating signals so as to substantially avoid an image component in each of the first and second output signals.